The invention is in the field of tuna siening nets which are designed to permit the escape of porpoises.
Present day tuna fishing techniques involve the use of a long net which is deployed by a boat called a "Siener". The net is extended in the water and the bottom weighted and the top edge floated so that it extends vertically downward from the water surface. The net is first deployed generally around a school of tuna and is drawn around to define an enclosure. The tuna ordinarily remain at a depth somewhat below the water surface and once the net is drawn completely around them the bottom edge of the net is drawn together or "pursed", and thus the nets are called "purse nets".
Once the net is pursed, there is no escape for the fish contained inside. Because porpoises are inclined to swim above a school of tuna, they are used as fish spotters and thus are often trapped in the nets as they are pursed closed. As the net is drawn tighter these mammals, in their frenzy to escape, will charge the net with such force that they will become trapped in the net fabric. Because the porpoises remain generally close to the water surface because they are air breathers, a net using a very fine mesh upper band has been devised and is quite effective in preventing the porpoises from becoming stuck in the net as a result of their charging.
Unfortunately, even though the porpoises are quite agile at jumping from the water and could easily jump over the cork line which floats the net if they tried, they are for some reason afraid to do this and instead will charge the net.
Current porpoise freeing techniques involve the use of a skiff, which the fisherman will take to a point along the net generally remote from the siener and then with a long pole or gaff will force portions of the net beneath the surface to permit porpoises to swim to their freedom. Another technique is to "back down" the siener which involves tugging the entire net through the water such that the rearmost portion dips beneath the surface. Occasionally, the fishermen have even been known to jump into the water and manually throw the porpoises over the net or through the depressed portion they have made in the net. This is somewhat time consuming and clearly it has not been completely effective or the current battle between the environmentalists and the fishermen, which has resulted in a crippling of the industry, would not have taken place.
Whereas the porpoise problem has spawned the development of a number of inventions designed toward providing a solution, nothing has been developed which is simple in terms of initial costs and operating man power, and at the same time is effective. The prior art also includes a net which operates on a principle similar to application's invention and is represented in U.S. Pat. No. 3,830,004. This device is a net which has an inflatable upper hose which can be kinked at various places with a long gaff to permit the deflation of certain portions, causing areas of the net to sink below the surface to permit the ingress of fish in response to the placement of bait within the net area. This is similar in principle to one of the features of applicant's invention.